Dimethyl ether (DME) is a valuable compound useful in the chemical industry, e.g. as precursor of dimethyl sulphate, acetic acid or for olefin production. It is an important research chemical and is used as refrigerant and propellant. Moreover, DME may find more widespread use in the future, as it is being developed as novel fuel, e.g. as replacement for or additive to propane in LPG and as diesel fuel additive. It can be produced by catalytic dehydration of methanol, and the methanol can be produced by catalytic hydrogenation of CO or CO2, e.g. using synthesis gas. The production of DME from synthesis gas may thus be accomplished via the direct or the indirect method. The indirect method involves contacting the synthesis gas with a methanol synthesis catalyst to form methanol, which is subsequently contacted with a dehydration catalyst to form DME. Alternatively, methanol could be used as a starting material which is contacted with a dehydration catalyst without the need of a methanol synthesis catalyst. The direct method involves contacting the synthesis gas with a bifunctional catalyst comprising a methanol synthesis catalyst and a dehydration catalyst, such that isolation and purification of the methanol is not required. Both the direct and the indirect method are presently commercially used for the production of DME.
Several reaction schemes for the synthesis of dimethyl ether have been developed, such as:Methanol synthesis: CO2+3H2↔CH3OH+H2O  (1)Water gas shift: CO+H2O↔H2+CO2  (2)Methanol dehydration: 2CH3OH↔CH3OCH3+H2O  (3)Overall: 3CO+3H2↔CH3OCH3(DME)+CO2  (4)
Two alternative overall reactions, based on reactions (1) to (3), for the synthesis of DME from synthesis gas are:2CO+4H2↔CH3OCH3(DME)+H2O  (5)2CO2+6H2↔CH3OCH3(DME)+3H2O  (6)
Typically, a direct DME catalyst system contains a methanol synthesis catalyst that is capable of catalysing reactions (1) and (2), and a dehydratation catalyst that is capable of catalysing reaction (3), although certain materials are known that are active in all three reactions. These latter materials are also referred to as bifunctional catalysts. For optimal carbon selectivity towards DME, it is generally preferable that DME is formed (mostly) via reaction (5) and/or (6). As such, the majority of the carbon atoms in the reactants end up in the desired product, i.e. DME, and not in a by-product such as CO2.
The interplay between the various reactions that together form the DME synthesis, such as between reactions (1)-(3) to give any one of overall reactions (4)-(6), is crucial for satisfactory DME yields, especially since all are equilibrium reactions. The CO to CO2 molar ratio—or in the context of the present invention the value of x in COx—plays a major role. Typically, small amounts of CO2 can be tolerated in the feedstock. However, since the removal of water by reaction (2) is crucial—after all, water is formed in both reaction (1) and (3) and is a by-product in overall reactions (5) and (6)—the CO2 content in the feedstock should be kept as low as possible. The presence of CO2 in the feedstock favours the reverse water gas shift reaction and thus shifts the equilibrium of reactions (1) and (3) towards the reactants. But even with pure CO as carbon oxide species, currently applied methods for the production of DME suffer from the major drawback that satisfactory yields of DME are only obtainable by use of major recycles. Herein, unreacted starting materials are separated from produced DME and rerouted to the reactor to be contacted again with the catalyst system. CO2 is a significant component of such recycles, but since only small amounts of CO2 can be tolerated by the bifunctional catalyst, it needs to be converted to CO before it can be recycled. Economically unfavourable conversion of CO2 into CO is needed, such as a dry reforming step. The equilibrium mixture also typically contains substantial amounts of methanol, which also need to be recycled to the catalyst in order to obtain an overall DME yield that is satisfactory. Such large recycles hamper the flexibility of the process and increase costs associated with e.g. keeping all reactant streams at the desired temperature and pressure. Moreover, large scale energy-consuming separations (e.g. distillations) are needed to isolated DME from the unreacted reactants and intermediates. WO 2005/026093 disclosed a process for producing DME wherein CO2 is converted into CO by a reverse water gas shift reaction, which process requires unfavourable separation of DME and CO2, as well as a large CO2 cycle. The indirect method for the synthesis of DME suffers from the same equilibria and need for recycles, with the only difference that reactions (1) and (2) take place in a different reaction zone as reaction (3). Hence, there remains a need in the art to increase the yield of DME without the need of major recycles, such that DME production may be more flexible and cost effective.
A further issue is the exothermicity of reactions (1)-(3). As these are all highly exothermic, a lot of heat is produced during DME synthesis. State of the art DME production facilities are able to cope with this heat, e.g. by using actively cooled slurry reactors to absorb the heat. As the indirect method for synthesizing DME distributes the heat formation over more than one reactor, this method is generally better capable to cope with the heat formation. The issues and challenges in state of the art DME synthesis are reviewed by Azizi et al. (Chem. Eng. Proc. 82, 2014, 150-172).
The use of an adsorbent which selectively binds water, to force the equilibrium of reactions (1) and (3) towards the products methanol and DME has been suggested by Iliuta et al. (Chemical Engineering Science 66 (2011) 2241-2251) and Hamidi et al. (Journal of the Taiwan Institute of Chemical Engineers 47 (2015) 105-112). Iliuta describes a model study for the use of a catalyst system comprising unspecified catalytic particles and zeolite-4 A as a third active material. Hamidi also reports a theoretical study, wherein a fixed bifunctional catalyst is combined with flowing zeolite-4 A particles which are capable of adsorbing water. Ressler et al. (in Integrated Reaction and Separation Operations, Modelling and experimental validation, editors: Henner Schmidt-Traub and Andrzej Gorak, Springer Verlag, 2006, ISBN10 3-540-30148-8, Chapter 4: Reactive gas adsorption) discloses a single-reactor DME synthesis starting from synthesis gas, using a 25:25:50 (by volume) combination of a methanol synthesis catalyst (Cu/ZnO/Al2O3), gamma-alumina and zeolite-3 A. In these disclosures, the catalyst system contains three distinct active materials, wherein the zeolite (3 A or 4 A) is used as a water adsorbent. Using a selective water adsorbent in the synthesis of DME is referred to as sorption-enhanced DME synthesis (SEDMES).
The present invention is concerned with providing a process and system for the production of dimethyl ether in a more efficient and cost-effective way. Most importantly, the need for large CO2 recycles and concurrent CO2-assisted reforming steps is obviated. Also, methanol recycles are kept at a minimum and issues with the exothermicity of the reactions are avoided. Lastly, the presence of CO2 in the equilibrium mixture is suppressed, which avoids the need for cumbersome separation of DME from a mixture comprising significant amounts of CO2 and largely eliminates the CO2 recycle, two of the most costly aspects of conventional DME production.